explain: typing-surface matrix cells (ADR-0028 step 5)

13 matrix cells for the `explain` prefix across all three
wrapped commands — `explain show data` / `explain update` /
`explain delete` — covering each typing position (after the
prefix, the inner entry word, the table, the filter clause)
plus the three complete forms. The cells confirm `explain`
plugs into the inner query grammars cleanly: candidates, hints
and column scoping match the standalone commands, and the
complete forms parse as `Command::Explain`.

Also adds a worker test pinning the display SQL's `<>`
rendering of inequality (ADR-0028 §3).

Matrix: 161 -> 174 cells. 1172 tests pass; clippy clean.

src/db.rs

@@ -8247,6 +8247,28 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn explain_display_sql_writes_inequality_as_angle_brackets() {
+        let db = db();
+        people_table(&db).await;
+        let plan = db
+            .explain_query_plan(parse_inner("show data People where Age != 35"))
+            .await
+            .unwrap();
+        // ADR-0028 §3: inequality is shown as standard SQL `<>`
+        // even when the user typed `!=`.
+        assert!(
+            plan.display_sql.contains("<>"),
+            "inequality should render as `<>`: {}",
+            plan.display_sql,
+        );
+        assert!(
+            !plan.display_sql.contains("!="),
+            "the `!=` spelling should not survive: {}",
+            plan.display_sql,
+        );
+    }
+
     #[tokio::test]
     async fn explain_of_a_missing_table_is_an_error() {
         let db = db();